Presently, circuit boards containing "surface-mounted" components, i.e., components which have conductive members (leads or pads) solder-bonded to corresponding metallized areas on the surface of the board, are fabricated in the following manner. First, a volume of solder paste is printed on the surface of the circuit board, or alternatively, a quantity of solder is applied to each conductive member of each component. Next, components are placed on the circuit board so that each conductive member on each component is in registration with a corresponding metallized area on the board surface. After placement, the component is adhered to the circuit board, either by the tackiness of the solder paste, or in the absence of any paste, by a conventional adhesive. Finally, the circuit board is heated, causing the solder paste on the board, or alternatively, the solder applied to the conductive member of each component, to reflow, thus creating a bond between the component and the circuit board.
Prior to the heating of the circuit board to reflow the solder or solder paste, it is useful to inspect the board to ensure that there are no missing or misaligned components. Repair of a circuit board having misaligned or missing surface-mounted components is far easier before the components are soldered in place than afterwards. However, in order for such "pre-solder" inspection to be useful, such an inspection must be carried out rapidly and with high accuracy.
Now-allowed U.S. patent application Ser. No. 111,954, filed in the name of I. Amir et al. on Oct. 23, 1987, and assigned to AT&T, discloses a system for inspecting a circuit board to detect missing and misaligned components. The inspection system disclosed in that application (incorporated by reference herein) is capable of reliably detecting missing or misaligned "passive" surface-mounted components (i.e., resistors and capacitors). However, the aforementioned inspection system does not reliably detect whether any active component, such as a plastic leaded chip carrier (plcc), soldered-on integrated circuit (soic), or soldered-on transistor (sot), is missing or misaligned.
Thus, there is a need for a technique for inspecting a circuit board to detect missing and misaligned passive and active surface-mounted components.